1. Field of the Invention
The present invention relates generally to a recording or printing head which is adapted to apply an electric current to an ink film or ribbon, or a recording medium, for effecting thermal printing of images such as characters on the recording medium. More particularly, the present invention is concerned with a multi-layer structure including a plurality of electrodes for locally energizing the ink film or recording medium.
2. Discussion of the Prior Art
Numerous proposals have been made in connection with an arrangement of electrodes in a recording or printing head. For example, laid-open Publication Nos. 61-35972, 58-12790, 61-230966, and 62-292461 of unexamined Japanese Patent Applications disclose a recording head having a multi-layer structure which includes a plurality of recording electrodes and a return circuit electrode (referred to as "common electrode") or a plurality of return circuit electrodes. More specifically, the recording and return circuit electrodes are held in contact with an ink film having an ink layer and an electrically resistive layer formed separately from the ink layer. The electrically resistive layer is locally energized to generate heat, by an electric current applied thereto by the electrodes of the recording head, so that a thermally fusible or vaporizable ink material on the corresponding local portions of the ink layer is softened or diffused due to heat of the electrically resistive layer, and then transferred to an ordinary recording medium. Thus, the printing of images is effected by the recording head. Alternatively, the ink film may have an electrically resistive ink layer which serves both as an ink layer and an electrically resistive layer. According to the structure of the recording head disclosed in the publications, an electrically insulating layer is disposed between an array of the recording electrodes and the return circuit electrode or an array of the return circuit electrodes.
In the recording head which incorporates the electrodes in its multi-layer structure as described above, it is required that the recording and return circuit electrodes are positioned with a high degree of accuracy so as to provide a constant spacing between each recording electrode and the return circuit electrode or electrodes. This is desired for preventing crosstalk between the electrodes, and for assuring consistent image transfer with enhanced reproducibility, namely, for obtaining transferred images consisting of a multiplicity of uniformly pointed dots.
Another type of recording head as disclosed in laid-open Publication No. 63-87264 of unexamined Japanese Patent Application is adapted to perform its printing operation on a heat-sensitive paper (recording medium). The heat-sensitive paper having both an electrically resistive layer and a coloring layer, or an electrically resistive coloring layer is directly held in contact with a recording surface of the recording head, while the heat-sensitive paper having only a coloring layer is used such that the recording head effects thermal printing of images on the paper through an electrically resistive film. Alternatively, an electrically resistive ink layer as indicated above is used, as indicated in laid-open Publication Nos. 58-104787, 61-37493 and 63-30279 of unexamined Japanese Patent Applications, in which the disclosed recording heads perform the thermal printing of images on an ordinary recording medium through the electrically resistive ink layer. In these publications indicated above, there is no description of the multi-layer structure of recording head in which the array of recording electrodes are spaced by a suitable distance from the return circuit electrode or electrodes, as described above. If such a multi-layer structure of recording head is used to effect the thermal printing of images on the heat-sensitive paper or through the electrically resistive ink layer, as indicated in the above publications, it is required that the recording and return circuit electrodes are highly accurately positioned so as to provide a constant spacing therebetween, as in the case described above.
To enable the electrodes of the recording head to be constantly held in contact with the electrically resistive layer or electrically resistive ink or coloring layer of the ink film or heat-sensitive recording medium, for local energization thereof, the insulating layer disposed between the recording and return circuit electrodes is required to be made of a material having a lower degree of wear resistance than that of the electrodes. The insulating layer also needs to have a considerably high degree of heat resistance, so as to prevent deterioration of its insulating property and a chronological change in the thickness of the layer, due to heat generated by the electrically resistive layer or electrically resistive ink or coloring layer, which lead to deterioration in the quality of printed or transferred images.
In the conventional recording head formed in a multi-layer structure including the recording and return circuit electrodes, however, the insulating layer disposed between the electrodes does not satisfy the latter requirement for heat resistance, since the insulating layer is for example formed of a resin material such as an epoxy resin or a polyimide.
In some conventional recording heads, the insulating layer disposed between electrodes is made of an ordinary glass or ceramic material. While such an insulating layer is satisfactory in its heat resistance, the wear resistance of the insulating layer is higher than that of the electrodes, resulting in an insufficient electrical contact of the electrodes with the electrically resistive layer of the ink film. Thus, it is difficult to obtain an optimum relative wear resistance between the insulating layer and the electrodes.
Where the electrically insulating layer is formed of mica, the layer has a high degree of heat resistance and sufficient insulating property. However, the use of ordinary mica for the insulating layer may cause insufficient uniformity in the thickness of the insulating layer, which determines a spacing between the recording and return circuit electrodes. Consequently, the recording head having such an insulating layer suffers from deterioration in the quality of the transferred images.